Speed sensors for aircraft engines and other applications routinely use electromagnetic, variable reluctance techniques. While such sensors are widely used and well-known but they are, however, quite bulky. There is a need for speed sensors which are substantially smaller and less expensive than the electromagnetic, variable reluctance schemes described above. Applicant has discovered an approach to measuring speed through a combination of a variable reluctance magnetic path and magneto-optic transduction in which a ferro magnetic tooth on the rotating member actuates a magneto-optic switch. The use of a magneto- optic switch in conjunction with a high energy, rare earth permanent magnet results in a compact and very effectively speed measuring system.
U.S. Pat. No. 4,818,080 (issued Apr. 4, 1989), entitled "Monolithic Faraday Optical Switch" assigned to the General Electric Company, the Assignee of the instant invention, describes a monolithic Faraday magneto-optic switch in which all of the switch functional elements viz, polarizers, analyzers, Faraday rotator layers, light reflecting surfaces are located on a single substrate. Faraday rotating layers are deposited on the surfaces of an optically inactive substrate. A polarizer-analyzer is deposited in a side-by-side arrangement on one side of the substrate and a non-magnetic, reflector surface is deposited on the other surface. Radiant energy from an input optical fiber is transmitted through the polarizer and through the Faraday rotational layer(s) and the optically inactive substrate to the reflecting surface where it is reflected back to the analyzer. The Faraday layer when subjected to a magnetic field which is orthogonal to its surface, rotates the plane of polarization of the incident radiation. Thus, the presence or absence of the magnetic field controls the optical switching function.
The monolithic-magneto-optic switch element is actuated whenever a magnetically permeable tooth or projection on a rotating member passes by the magnet. The magnet and the switch are so oriented with respect to each other that in the absence of a tooth, there is no magnetic field component at the surface of the switch Faraday layer(s). Whenever a tooth passes by the magnet, the magnetic field is distorted causing the magnetic field vector to shift so that a magnetic field component now exists along the sensitive axis of the magneto-optic switch. This rotates the plane of polarization of the incident light and light passes through the switch.
It is therefore a principal objective of the invention to provide a speed sensor utilizing a Faraday magneto-optic transducing arrangement.
It is a further objective of the invention to produce a small, light weight speed sensor utilizing a Faraday magneto-optic switch.
Yet another objective of the invention is to provide a speed sensor in which passage of the rotating member generates a magnetic field component which actuates a magneto-optic switch to produce an output signal.
Other objectives and advantages of the invention will become readily apparent as the description thereof proceeds.